In automotive applications, electrical loads, such as lamps, motors, solenoids and the like, are typically controlled by an electrical switch, such as a field-effect transistor (FET) or a relay. Such switches are often grouped together in a module and are selectively (de)activated by a pre-drive or control integrated circuit (IC), which can be embodied as a custom application specific integrated circuit (ASIC), a microcontroller (μC), or can be integrated into the FET itself. Switches must allow enough current to flow to the loads during normal operation and prevent current from flowing in the event of an over-current or over-temperature event that may destroy the FET, the load, or the associated vehicle wiring.
In addition to self protection, a vehicle engine control system needs an accurate measurement of FET current to ensure that loads used in safety related applications are functioning properly.
Several distinct types of FETs can be applied for switching applications. Standard FETs are three terminal devices, including a source, drain and gate. In order to sense the drain to source current (Ids) from a standard FET, at least one additional current sensing element is required, such as a current sensing resistor placed in series with the load current from which the resistor voltage is monitored. Current and temperature sensing FETs (sense FETs) are similar to standard FETs but have one or two additional features. First, sense FETs have one or more additional source terminals which carry a small fraction of the load current which can be monitored by an external control circuit to determine overall Ids. Second, sense FETs have one or more integrated temperature sensing diodes which sample the FET junction temperature (Tj). Smart FETs are similar to sense FETs but also have the ability to protect themselves from over-current and over-temperature events without an external control IC. Sense FETs and smart FETs typically have sufficient current sensing accuracy for use in automotive safety related applications, but are substantially more expensive than standard FETs. Even though multiple sense FETs and smart FETs are routinely incorporated within a common package to save board space, their cost remains problematic.
It has been suggested that Ids can be determined for standard FETs without the need of an external current sensing element by measuring the FET Vds and by assuming a value for the on-resistance (Rds) of the FET. However, Rds varies significantly with temperature and thus is expressed as a function of Tj as Rds (Tj). In a typical FET, for example, Rds(175C) is twice the value of Rds(25C). Assuming the FET is off and at ambient temperature (Ta), the application of Ids which flows through Rds will cause a power (Ids2*Rds(Tj)) to be dissipated by the FET. The heat generated by this power will primarily flow through a thermal resistance between the FET junction and case (Rth,jc) and subsequently through a thermal resistance between the FET case and ambient (Rth,ca), where Rth,ca >>Rth,jc and thus Rth,jc can typically be neglected. Accordingly, the heat flow that determines Tj can be described by the following equationTj=Ta+Ids2*Rds(Tj)*Rth,ca   (1)
Tj and thus Rds are dynamic and in general unknown unless they are directly measured or calculated. Furthermore, Rds is generally a non-linear function of Tj such that solving (1) will in general require mathematical iteration. These facts discourage the estimation of Ids for standard FETs by measurement of Vds if Tj and thus Rds are not independently measured. By analogy, it has been suggested that the value of Tj can be determined for a standard FET by measuring Vds and Ids and calculating Rds which can then be used to determine the value of Tj but this requires that Ids be measured separately from Vds and thus requires the addition of an external current sensing element, thus increasing the cost.
As a further complication, it is desirable to place multiple FETs in a single package having a common drain terminal and thus common case terminal. Since the FETs have a common case, heat generated from each FET will flow through a common thermal path from case to ambient and the junction temperature of each FET will be interdependent. The junction temperature Tj for “n” non-parallel FETs in a common case is described by the following equation (sum over i=1 to n):Tj=Ta+Σ{Ids(i)2*Rds(i,Tj)}*Rth,ca;   (2)
By analogy with (1), (2) is in general a non-linear function requiring mathematical iteration to solve Tj to high accuracy.
Prior art U.S. Pat. No.: 7,154,291 B2 to Turner entitled “Measuring Bi-Directional Current Through A Field-Effect Transistor By Virtue Of Drain-To-Source Voltage Measurement” describes a method and apparatus for measuring Ids, and particularly bi-directional current, in a field-effect transistor (FET) using Vds measurements.
Prior art U.S. Pat. No.: 4,896,245 (Qualich) entitled “FET Overtemperature Protection Circuit”, prior art U.S. Pat. No. 7,248,452 (Ohshima) entitled “Method of Protecting Semiconductor Device and Protection Apparatus for Semiconductor Device using the same”, and prior art U.S. Pat. No. 6,107,669 (Mokuya) entitled “Load Actuating Semiconductor Circuit Having a Thermally Resistive Member” all describe methods and apparatus for detecting FET Tj by monitoring Vds for a known Ids.
The specification and teachings of U.S. Pat. Nos. 7,154,291 B2, 4,896,245, 7,248,452, and 6,107,669 are hereby incorporated herein be reference.
Existing standard FET solutions which use only the measured value of Vds and known temperature dependence of Rds(Tj) to estimate Ids and Tj have insufficient accuracy for automotive safety applications because they either assume a value for Tj leading to a value for Rds which is then used to estimate Ids or they assume a value for Ids which is then used to determine a value for Rds leading to a value for Tj. Since Tj and Ids can vary significantly from the assumed value, the accuracy of these approaches can have significant error, especially when multiple FETs are thermally coupled by virtue of being placed on a common case.
Accordingly, what is required is a method for determining the Ids and Tj with high accuracy, for one or more thermally-coupled or uncoupled standard FETs, without the use of additional current or temperature sensing elements. The values of Ids and Tj once determined, can then be used to protect the devices against over current or over temperature stress.